1. Field of the Invention
This invention relates to parting material that is interposed between the facing surfaces of a pair of matched glass sheets that are bent simultaneously at elevated temperatures while supported in bending relation to a mold for bending glass sheets.
2. Technical Problems and Prior Art
When glass sheets are shaped in pairs to the shape desired for laminated windshields, it is necessary to separate the sheets after bending and assemble them as a sandwich with a plastic interlayer therebetween. Unless a suitable parting material is interposed between the glass sheets, the elevated temperature needed to sag the glass sheets by gravity to conform to a shaping mold or to simultaneously press bend overlaying glass sheets may cause the glass sheets to stick together during their shaping, making their separation without resultant glass damage impossible. This precludes the insertion of an interlayer between the bent glass sheets for final lamination.
Prior to this invention, natural silica deposits in the form of diatomaceous earths identified by the trade name CELITE.RTM. available from Manville Product Corporation, Colorado, were milled and mixed in an aqueous slurry and applied by spray to a major surface of one of the glass sheets of a pair to bent simultaneously in a manner well known to those skilled in the art. The diatomaceous earth particles kept the glass sheets in closely spaced relation as they sagged in unison to conform to the shape of the glass sheet shaping mold. However, diatomaceous earth particles have a random distribution of size and are hard in nature which, as the prior art does not appreciate, may cause a defect in the form of small round spots, or optical distortions commonly termed as a bull's-eye, that appear in the laminated windshields.
Also prior to this invention, amorphous precipitated silica was available as a flatting agent for paint and for manufacturing microporous battery separators. U.S. Pat. Nos. 4,095,994 and 4,263,051 to Crawford et al. recite various silica compositions useful as flatting agents for paints and U.S. Pat. No. 4,681,750 to Johnson et al. recites amorphous, precipitated silica compositions useful for manufacturing microporous battery separators and methods of preparing such silica compositions.
There is no suggestion in these patents of using precipitated silica compositions as parting materials between glass sheets undergoing simultaneous bending by exposure to heat-softening temperature of said glass sheets.